Polymerization of propylene is usually carried out in the presence of a catalyst system containing a Ziegler-Natta type procatalyst, an organo aluminium cocatalyst, an internal electron donor and a selectivity control agent. Depending upon the kind of internal electron donor used, the polymerization catalyst systems could either be (a) a monoester based catalyst system or (b) a diester based catalyst system.
Monoester based catalyst systems use monoester of aromatic carboxylic acids as internal electron donors. When a monoester based catalyst system is used, the polypropylene productivity is usually low to moderate and the polymers having relatively high polydispersity are formed. However, the extent of kinetic control achieved for the polymerization reaction, using such catalyst systems, is high, thus avoiding run away reactions leading to chunk formation. In most cases, monoester based catalyst systems provide polymerization reactions that are self extinguishing in nature i.e polymerization activity reduces with increasing temperature of the reaction.
Diester based catalyst systems typically use diester of aromatic carboxylic acids as internal electron donor. Polymer productivity using such catalyst systems is high. When diester based catalyst systems are used, polymers are usually formed with low polydispersity. It is, however, difficult to achieve adequate kinetic control of the polymerization reaction using such catalyst systems.
U.S. Pat. No. 5,414,063 describes a process for polymerization of propylene using a diester based catalyst system wherein a silane compound is used as the selectivity control agent. The process uses p-ethoxy ethyl benzoate as a killing agent to terminate the polymerization reaction. The use of killing agents to terminate the polymerization reaction would require that the polymerization reaction be continuously monitored either manually or by an automated control system. It is desirable that the polymerization reaction is terminated by itself so that such manual/auto controlled monitoring of the reaction progress could be avoided